Broadband transmission of signals has been in used for a number of years. For example, cable television systems utilize broadband transmission of signals to deliver various content (e.g., video, audio, and data) using a plurality of discrete channels. Many modern cable systems transmit a large number of discrete channels (e.g., 155 channels), each occupying a predetermined bandwidth (e.g., 6 MHz or 8 MHz channel bandwidth), using broadband spectrum (e.g., approximately 50 MHz-1 GHz). Accordingly, various content including broadcast content (e.g., television programs, movies, and audio programs), controlled delivery content (e.g., pay-per-view programs), on-demand content (e.g., individually requested/initiated program delivery), session content (e.g., individual data communication sessions), etc. may be provided through a cable system for selective use by subscribers and/or other receiver points.
Directing attention to FIG. 1, an exemplary cable system is illustrated as cable system 100. Cable system 100 includes cable transmission node 101 coupled to transmission media 102. Cable transmission node 101 may comprise a cable plant head-end, a repeater, or various other nodes utilized in transmission of cable system signals (e.g., 155 channels transmitted in the spectrum from approximately 50 MHz-1 GHz). Although referred to as a transmission node, it should be appreciated that cable systems may be bi-directional (e.g., providing some up-link capability such as for data sessions, interactive programming, etc.) and thus cable transmission node 101 may accommodate transmission and reception of signals. Transmission media 102 couples cable transmission node 101 to a plurality of receiver points, shown here as subscriber premises 111-113. Various devices, such as amplifier 103, may be provided in the cable system to facilitate desired signal communication.
Receiver circuitry may be utilized at the subscriber premises to select particular signals of the broadband transmission and to obtain content there from. Receiver circuit 200 of FIG. 2 shows a receiver circuit configuration which may be utilized at any or all of subscriber premises 111-113 to select a desired signal and obtain content carried by the selected signal. Circuit input 201, such as may be coupled to a tap or “drop” from transmission media 102, provides the signals of the broadband transmission to receiver circuit 200. Filter 211, such as may comprise a band-pass filter, provides signal filtering to substantially exclude signals outside of the spectrum of the broadband transmission (e.g., <50 MHz and/or >1 GHz). Amplifier 221, such as may comprise a low noise amplifier (LNA), provides signal amplification and/or buffering to compensate for noise and facilitate an acceptable noise figure with respect to receiver circuit 200. Mixer 231 provides frequency translation from a radio frequency (RF) used for converting a selected signal (or a contiguous block of signals) to a baseband frequency used in demodulating the selected signal. Filter 212, such as may comprise a low-pass filter, provides signal filtering to substantially exclude signals above the frequency translated selected signal (narrow-band demodulating receiver) or block of signals (wide-band demodulating receiver). Data converter 214, such as may comprise an analog to digital converter (ADC), converts the selected signal or block of signals to a digital representation for digital processing by digital signal processor (DSP) 251.
It should be appreciated that operation of cable system 100 introduces undesired attributes to the transmitted signals. For example, noise, signal distortion, and group delay may be introduced to signals transmitted by cable system 100. Transmission media 102 may provide a frequency response whereby different portions of the broadband transmission spectrum are affected differently by distortion. For example, coaxial cables typically utilized in cable transmission systems such as cable system 100 provide a frequency response in which higher frequencies are attenuated more so than lower frequencies. That is the transmission media rolls off at the high frequencies and thus higher frequencies experience greater attenuation as they propagate through the transmission media than do low frequencies. Broadband transmission in such a system experiences frequency response “tilt”. Such frequency response tilt can be significant over a wide frequency range, such as the approximate gigahertz range common to cable transmission systems.
Various components of cable system 100 may be adapted to compensate for and/or mitigate the introduction of undesired attributes to the transmitted signals. For example, cable transmission node 101 and/or amplifier 103 may be adapted to provide offsetting tilt with respect to the broadband transmission in order to compensate for the aforementioned high frequency attenuation. As shown by graph 121 of FIG. 1, the amplitude of the broadband transmission signal may receive a positive amplitude tilt such that signals at the higher frequency end of the spectrum are transmitted at a greater amplitude. This positive amplitude tilt may be utilized to offset frequency response tilt, at least to some extent, associated with broadband transmission of signals. Nevertheless, transmission through media of sufficient length may overcome the positive amplitude tilt, resulting in a negative amplitude tilt of the broadband transmission signal, as shown by graph 123 of FIG. 1.
As illustrated by graphs 121 and 123 of FIG. 1, receiver circuits of subscriber premises disposed at different points in the cable system may experience appreciably different received signals. For example, receiver circuit 200 of subscriber premise 111, disposed nearest to cable transmission node 101, may experience received signals (graph 121) having not only relatively little attenuation from propagation through the cable transmission system, but also having the positive amplitude tilt provided to compensate for frequency response tilt of transmission media 102. In contrast, receiver circuit 200 of subscriber premise 113, disposed furthest from cable transmission node 101, may experience received signals (graph 123) having not only appreciable attenuation from propagation through the cable transmission system, but also having negative amplitude tilt resulting from the frequency response tilt of transmission media 102.
Accordingly, measurement of signal levels as received at various locations, such as subscriber premises, may be utilized by devices of cable system 100. For example, cable transmission node 101 may obtain power reports from subscriber premises devices for use in controlling aspects of the broadband transmission of signals to improve or optimize operation of cable system 100. Thus, DSP 251 may operate to provide power measurement processing with respect to a digital representation of a signal provided thereto for reporting to cable transmission node 101 and/or other devices of cable system 100.
In order to provide information regarding the power level of the channels of the broadband transmission of signals as received by receiver circuit 200 (i.e., the channel power level at circuit input 201), DSP 251 of typical prior art embodiments operates to correct the channel power level as measured by DSP 251 to account for the effects of components (e.g., filter 211, amplifier 221, mixer 231, filter 212, and data converter 214) of receiver circuit 200. For example, DSP 251 may operate to correct a channel power measurement to subtract a gain factor of a receiver circuit amplifier, add a signal loss amount associated with a receiver circuit filter, etc.
The specifications for many cable transmission systems impose relatively precise requirements upon the measurement of received signal power levels. For example, some DOCSIS specifications impose a ±3 dB measurement requirement for received signal power level reporting. Thus, manufacturers must generally provide extensive calibration with respect to receiver circuits, such as to measure the frequency response for many receiver circuit components over all or substantially all of the broadband spectrum, in order to facilitate accurate correction of the channel power level as measured by a receiver circuit. A lookup table, for example, may be generated through such calibration for use by DSP 251 in applying appropriate corrections to a power level measurement for a particular channel in order to provide a received signal power level report of sufficient accuracy. Such extensive calibration and its attendant lookup table present a significant cost, time, and resource utilization challenge with respect to providing acceptable received signal power level measurement in the prior art.
Additional challenges are posed in the heretofore prior art implementations. For example, DSP 251 of receiver circuit 200 is operable to provide signal demodulation of selected signals provided thereto for utilization of content by a user or termination device. Accordingly, where a power level measurement is needed for a signal of a channel other than that currently selected for demodulation (i.e., translated to baseband by mixer 231 and passed by filter 212), interruption of demodulation of a desired channel may be necessary in order to perform received signal power level measurement. This may be true even where multiple signals (e.g., a contiguous block of channels) are converted to baseband.
The need for interruption of desired channel demodulation is illustrated in FIGS. 3A (illustrating the received channel profile at subscriber premise 111) and 3B (illustrating the received channel profile at subscriber premise 113), wherein channel 301 is a currently selected channel being demodulated by DSP 251 and channel 302 is a channel for which received signal power level measurement is desired. Where receiver circuit 200 provides for baseband processing of a single channel, as indicated by pass-bands 310a and 320a, demodulation of channel 301 must be interrupted in order to provide channel 302 to DSP 251 for power measurement processing. Similarly, even where receiver circuit 200 provides for baseband processing of multiple channels (e.g., a contiguous block of 4 channels), as indicated by pass-bands 310b and 320b, demodulation of channel 301 must be interrupted in order to provide channel 302 to DSP 251 for power measurement processing. However, interruption of channel demodulation may result in loss of data, interruption in service, and other undesirable and/or unacceptable consequences: